This invention relates to a resin composition containing an aromatic episulfide. The resin composition is used as a liquid resin molding material which exhibits excellent moldability required for sealing electrical parts, electronic parts and semiconductor chips by potting or casting and excellent moisture resistance after curing. Furthermore, the resin composition is used as optical materials such as plastic lenses, prisms, optical fibers, optical films and filters, adhesives for the aforementioned optical materials and coatings and also used in sealing light-emitting devices and light sensors.
With a rapid growth of electronics in recent years, semiconductor devices such as IC and LSI are used in various fields and a trend to lower cost and higher integration has brought forth a variety of new packaging schemes; dual in-line package by transfer molding with the use of a conventional mold is being replaced by moldless spot sealing of bare chips such as hybrid IC, chip on board, tape carrier package and plastic pin grid array. Curing agents for these liquid epoxy resin molding materials include amines such as dicyandiamide and dihydrazideaminimide compounds and liquid acid anhydrides such as hexahydrophthalic acid anhydride, tetrahydrophthalic acid anhydride and methylhimic anhydride. They were defective, however, as the former were strongly polar to lower the bias properties while hydrolyzed extensively in the pressure cooker test (PCT) and suffered from deterioration of adhesiveness after absorption of moisture. For this reason, an attempt was made to use solid novolac type phenolic resin as a curing agent, but the molding material became highly viscous with some loss in moldability and, to its disadvantage, required the simultaneous use of an organic solvent.
In the cases where liquid epoxy resin such as bisphenol A type epoxy resin and alicyclic epoxy resin is cured by an acid anhydride curing agent for ease of molding a composition, the optimal epoxy resin to acid anhydride ratio which gives the highest glass transition temperature and the lowest water absorption is decided (for example, epoxy group/acid anhydride group=1.0/0.75-1.0/0.9 on a mole basis in case a tertiary amine catalyst is used); still the water absorption is high in excess of 2% and the refractive index is unsatisfactorily 1.55 or less.
Novel alkylsulfide type episulfides, their compositions and cured products are proposed in JP9-71580 A(1997) and JP9-110979 A(1997). An alkylsulfide type episulfide cures by the use of an amine catalyst into a good optical material with a softening point of 100xc2x0 C. or more, a refractive index of 1.69 or more and an Abbe number of 35 or more. In the patents, an explanation is found of a composition which uses a primary amine or an acid anhydride as a curing agent; however, an example in which a primary amine is used gives a cured product with a low softening point of 100xc2x0 C. or less and records no concrete data such as refractive index and water absorption regarding the cured product by the use of an acid anhydride and the effect of the curing agent used is not clear. Moreover, a compound containing a mixture of epoxy and episulfide groups presents a problem of curing with low heat resistance and/or low strength and no concrete description is found of a composition containing this kind of compounds and an acid anhydride as a curing agent and of the properties of cured product.
Nishikubo et al. have reported that a combination of a thioester initiator and a quaternary ammonium salt catalyst is the most active in the polymerization of episulfide compounds; refer, for example, to Polymer Journal, 28(1), pp 68-75, 1996 or Prpg. Polymn. Sci, Vol. 18, pp 963-995, 1993. J. P. Bell et al. are conducting research on the curing reaction involving an episulfide compound and a primary amine. No literature in public knowledge refers to the curing reaction involving an episulfide compound and an acid anhydride curing agent.
Transparent resins are attracting attention as optical materials and they are used in optical lenses, films, prisms and base materials for optical disks because of their lightweight, impact resistance and ease of molding. Moreover, they are used extensively in optical fibers for utilization of their optical properties and also used as manufacturing aids such as sealing materials for optical disks, light-emitting devices and light sensors, coatings, and adhesives.
Resins which are practically used in optical applications at the present time include poly(methyl methacrylate), poly[diethylene glycol bis(aryl carbonate)], polystyrene and polycarbonate as optical lenses and poly(methyl methacrylate) and poly[diethylene glycol bis(aryl carbonate)] as lenses for correction of faulty vision.
However, both poly(methyl methacrylate) and poly[diethylene glycol bis(aryl carbonate)] show a low refractive index on the order of 1.50. When the methacrylic resin is used as a lens for correction of faulty vision, it suffers from the defect of the thickness of lens at the edge becoming greater compared with a lens of inorganic glass and it has additional weakness as it is apt to absorb moisture, deform and change in refractive index and shows a heat resistance of 100xc2x0 C. or less. On the other hand, the polycarbonate is high in both refractive index and heat resistance, but it shows a defective tendency to absorb moisture and deform.
In those devices which utilize light such as optical disks, optical fibers, optical films for use in liquid crystal displays and light-emitting devices, operations such as pasting, joining and sealing are performed by the use of transparent resins as adhesives, coatings and sealing materials. The properties required for these optical materials include not only transparency but also low birefringence, low moisture absorption, high heat resistance, good quality for precision molding and matching in refractive index to components to be joined (refer, for example, to JP10-67977 A(1998) and Polyfile, July issue, p. 28, 1999).
Transparent resins of thermal crosslinking type are proposed in consideration of the aforementioned points and heat-curable optical materials possessing a thiourethane structure obtainable from the reaction of a polythiol and a polyisocyanate are proposed in JP4-58489 B(1992), JP5-148340 A(1993) and JP10-120676 A(1998). However, the materials in question are defective in that the pot life from mixing of the resin components to molding is short and no reference is found in these patents to lowering of water absorption.
An object of this invention is to provide a liquid resin molding material suitable for sealing semiconductors which is liquid before curing, cures with a high rate with good productivity, possesses after curing a glass transition temperature higher than the pressure cooker test (PCT) temperature of 121xc2x0 C., and is characterized by high heat resistance, low moisture absorption, good workability and excellent moisture resistance reliability after packaging.
Another object of this invention is to provide a curable resin composition which is characterized not only by transparency but also by low moisture absorption, high heat resistance, quality for precision molding and matching in refractive index to components to be joined and to provide the cured composition as an optical material.
This invention relates to a resin composition which comprises an aromatic episulfide compound (component A) having two or more reactive groups represented by the following formula (1) 
(wherein X is O or S with S accounting for 50 mol % or more of X on the average and R1-R4 are independently a hydrogen atom, a halogen atom or an alkyl group with 1-4 carbon atoms) in its molecule, at least one kind of glycidyl compound (component B) selected from an aromatic glycidyl ether compound (B1) having two or more glycidyl groups in its molecule and a glycidyl ester compound (B2) having two or more glycidyl groups in its molecule, an acid anhydride (component C) and a curing catalyst (component D) as essential components and satisfies the following criteria regarding the proportion of functional groups in the components and the amounts of the components; the total of the glycidyl and xcex2-epithiopropyl groups is 1.35-3.5 moles, the amount of the xcex2-epithiopropyl group is 0.5-2.2 moles and the amount of the glycidyl group is 0.5-1.9 moles per 1 mole of the acid anhydride group and the amount of component D is 0.01-5 parts by weight per 100 parts by weight of the total of components A, B and C.
Moreover, this invention relates to the aforementioned resin composition wherein the total of the glycidyl and xcex2-epithiopropyl groups is 1.35-3.5 moles, the amount of the xcex2-epithiopropyl group is 0.5-2.2 moles and the amount of the glycidyl group is 0.5-1.6 moles per 1 mole of the acid anhydride group.
Still more, this invention relates to a cured product which is obtained by curing the aforementioned resin composition by polymerization and useful as an optical material with a refractive index of 1.5 or more and a water absorption (saturated water absorption at 85xc2x0 C. and 85% RH) of 1% or less.
Aromatic episulfide compounds (component A) having the reactive group represented by formula (1) are prepared by a known method from known aromatic glycidyl ether compounds (also known as epoxy resins). Such aromatic glycidyl ether compounds include aromatic glycidyl ether compounds having two or more glycidyl ether groups in the molecule obtained by the reaction of epichlorohydrin with bisphenols such as bis(4-hydroxyphenyl)ketone, bis(4-hydroxyphenyl)sulfone, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)ether, bis(4-hydroxyphenyl)hexafluoropropane, 9,9-bis(4-hydroxyphenyl)fluorene, bis(4-hydroxyphenyl)dimethylsilane, 4,4xe2x80x2-biphenol and tetramethyl-4,4xe2x80x2-biphenol, polyfunctional phenols such as phenolic novolac, cresolic novolak, naphtholic novolac and condensation products of naphthol or naphthalenediol with 1,4-xyleneglycol, or the foregoing phenols whose hydrogen atoms on the aromatic rings are substituted wholly or partly by halogen atoms and alkyl groups with 1-4 carbon atoms. They may be used singly or as a mixture.
A compound containing the episulfide group is prepared by treating one of the aforementioned glycidyl ether compounds with a thio compound such as a thiocyanate, thiourea, triphenylphosphine sulfide and 3-methylbenzothiazole-2-thione, preferably with a thiocyanate or thiourea, thereby converting the glycidyl group partly or wholly to a thiuronium salt. The thio compound here is used stoichiometrically in an amount equimolar to or greater than that of the epoxy group and the use of a slight excess is desirable in consideration of product purity and reaction rate. On the other hand, if the objective is the partial conversion of the glycidyl group in a glycidyl ether compound to the xcex2-epithiopropyl group, the use of less than an equimolar amount poses no problem. If the preparation of a resin composition capable of accomplishing the object of this invention is taken into consideration, it is necessary to convert the epoxy group to the xcex2-epithiopropyl group at a rate of 50 mol % or more and, in turn, it is necessary to use a thio compound in an amount equal to xc2xd or more of the theoretical amount on a mole basis.
The reaction can be carried out either solventless or in a solvent. In case a solvent is used, the reaction is desirably carried out in a heterogeneous system by dispersing a thio compound or an aromatic glycidyl ether compound finely in a solvent or a solvent capable of dissolving either of the compounds is used in order to improve the yield of the target compound. Concrete examples of solvents are water, alcohols such as methanol, ethanol and isopropanol, ethers such as diethyl ether, dioxane and diglyme, aromatic hydrocarbons such as benzene, toluene and xylene and halogenated hydrocarbons such as chloroform and chlorobenzene. It is possible to carry out the reaction in a mixed solvent, for example, in two phase by a combination of water and an aromatic hydrocarbon. In this case, it is possible to wash out the unreacted glycidyl ether compound at the same time.
It is desirable to add an acid as a reaction promoter to the reaction mixture. Examples of such acids are nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid and propionic acid and these acids may be used together. The amount to be added is 0.1-20 wt % of the total reaction mixture. The reaction temperature is normally 20-100xc2x0 C. and the reaction time is usually 20 hours or less. As the intermediate product is usually obtained as solid, it is collected by filtration, washed with a solvent capable of dissolving the raw material aromatic glycidyl ether compound such as toluene to remove the unreacted raw material and then washed further with water until the pH of the washings becomes 3-5. The intermediate thus obtained is pulverized and dispersed in an excess of aqueous sodium carbonate or potassium carbonate at 20-70xc2x0 C. for 2-20 hours. The solid reaction product is then washed with water, dried and dissolved in an organic solvent such as toluene and the insoluble unreacted salt is filtered off to obtain a solution of the target aromatic episulfide. The target compound can be obtained by stripping the solvent from this solution.
The glycidyl compound having two or more glycidyl groups in its molecule (component B) to be used in this invention is at least one kind selected from an aromatic glycidyl ether compound (B1) and a glycidyl ester compound (B2). In order to obtain a resin composition as liquid, the selected components B1 and B2 are desirably liquid. Component B1 is preferred for the purpose of obtaining a composition of lower water absorption.
Known aromatic glycidyl ether compounds can be used as B1 and they include concretely bisphenol type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin and novolac type epoxy resin. It is also possible to use glycidyl ether type epoxy resin similar to the one which serves as a raw material of the aforementioned component A.
Known glycidyl ester compounds can be used as B2 and they are concretely diglycidyl phthalate, diglycidyl hexahydrophthalate and diglycidyl tetrahydrophthalate. They may be used singly or as a mixture of two kinds or more. Moreover, these glycidyl compounds are preferably liquid and those glycidyl compounds which are solid at room temperature can advantageously be blended by melting under heat and made liquid as a mixture.
Any known acid anhydride useful as a curing agent can be used as the acid anhydride (component C) of this invention and concrete examples of such acid anhydrides are alicyclic acid anhydrides such as methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylhimic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride and methylcyclohexenedicarboxylic acid anhydride, aromatic acid anhydrides such as phthalic anhydride, trimellitic anhydride, pyromellitic dianhydride, benzophenonetetracarboxylic acid dianhydride, ethylene glycol bis(trimellitate) dianhydride and glycerol tris(trimellitate) trianhydride, and halogenated acid anhydrides such as HET acid anhydride and tetrabromophthalic anhydride. Liquid alicyclic acid anhydrides are preferable from the viewpoint of transparency and ease of molding and they may be used singly or as a mixture of two kinds or more. It is advantageous to render a mixture liquid by mixing the selected acid anhydride with low-melting hexahydrophthalic anhydride.
A curing catalyst (component D) is used in this invention for the purpose of curing the composition by heating. It is possible to use any curing catalyst known for the epoxy resin/acid anhydride curing system and what is useful here is the one which gives the target cured product when mixed with the aforementioned components A, B and C and heated at 50-200xc2x0 C., preferably at 50-180xc2x0 C., more preferably at 80-180xc2x0 C.
Curing catalysts are exemplified by tertiary amines, phosphines, quaternary ammonium salts and Lewis acids. Concrete examples are tertiary amines such as triethylamine, triphenylamine, tri-n-butylamine, N,N-dimethylaniline and pyridine, imidazole and its derivatives such as imidazole, N-methylimidazole, 2-methylimidazole, 4-methylimidazole and 1-benzyl-2-methylimidazole, diamines such as 1,8-diazabicyclo(5,4,0)undecene-7,1,5-diazabicyclo(4,3,0)nonene and 5,6-dibutylamino-1,8-diazabicyclo(5,4,0)undecene-7, adducts of tertiary amines represented by the foregoing with organic acids, quaternary ammonium salts resulting from the aforementioned amines and halogens, Lewis acids, organic acids, mineral acids and tetrafluoroboric acid, phosphines such as triethylphosphine, triphenylphosphine and tri-n-butylphosphine and Lewis acids represented by boron trifluoride and boron trifluoride etherate. Of these examples, imidazoles and phosphines are preferable from the viewpoint of reliability of semiconductor devices while microencapsulated latent curing agents of the two are preferable from the viewpoint of pot life. In case an optical material is intended, imidazoles and quaternary ammonium salts, preferably the latter, are desirable because they give cured products of reduced coloration.
The aforementioned compounds may be used singly or as a mixture of two kinds or more. The curing catalyst or component D is used in an amount of 0.01-5 parts by weight, preferably 0.01-3 parts by weight, more preferably 0.1-2 parts by weight, per 100 parts by weight of the total of components A, B and C. Water absorption of the cured product increases when the amount of curing catalyst is more than 5 parts by weight while curing occurs insufficiently with unsatisfactory heat resistance when the amount is less than 0.01 part by weight.
It is allowable to add a thioester compound or a mercaptan compound as an initiator to the resin composition of this invention and thioester compounds boiling at 100xc2x0 C. or more are preferable as they give the composition a longer pot life and reduce coloration of the cured product. Concrete examples of mercaptan compounds are 2-mercaptoethanol, ester-containing aliphatic mercaptan compounds such as 2-ethylhexyl thioglycolate and 2-ethylhexyl 3-mercaptopropionate and polymercapto compounds such as trimethylolpropane tris(xcex2-thiopropionate) and pentaerythritol tetrakis(xcex2-thiopropionate). Examples of thioesters are S-phenyl thioacetate and thioacetates and thiobenzoates of the aforementioned mercaptan compounds.
The initiator is added normally in an amount of 0.01-5 parts by weight, preferably 0.05-3 parts by weight, more preferably 0.05-2 parts by weight, per 100 parts by weight of the total of components A, B and C. The pot life of the composition shortens and the heat resistance of the cured product deteriorates when the initiator is added in an amount in excess of 5 parts by weight.
In order for the cured resin composition of this invention to exhibit both low water absorption and good heat resistance or these two properties plus high refractive index, the composition of components A, B and C is so formulated as to obtain the following proportion of the functional groups in the components; the total of the glycidyl and xcex2-epithiopropyl groups is 1.35-3.5 times, preferably 1.5-3.2 times, the amount of the acid anhydride group on a mole basis while the amount of the xcex2-epithiopropyl group is 0.5-2.2 times, preferably 0.5-2.0 times, and the amount of the glycidyl group is 0.5-1.9 moles, preferably 0.5-1.6 times, the amount of the acid anhydride group on a mole basis. More preferably, the amount of the glycidyl group is 0.5-1.3 times in case the liquid moldability is important or 0.5-1.5 times in case the optical characteristics are important.
Component A can have the glycidyl and xcex2-epithiopropyl groups depending upon the kind of X in general formula (1). Component B has the glycidyl group and component C has the acid anhydride group [xe2x95x90(CO)2O]. A compound whose one mole contains n acid anhydride groups is defined as n moles of acid anhydride groups. Likewise, a compound whose one mole contains n glycidyl groups is defined as n moles of glycidyl groups and a compound whose one mole contains n moles of xcex2-epithiopropyl groups is defined as n moles of xcex2-epithiopropyl groups. When a compound has both glycidyl and xcex2-epithiopropyl groups, the number n is computed by proration.
Compounds having xcex2-epithiopropyl groups are generally solid at room temperature; solventless compositions containing them are not easy to handle alone and form brittle films when applied as coatings. On the other hand, curing of the glycidyl group alone does not give a product of sufficiently crosslinked structure. When a compound having a mixture of the xcex2-epithiopropyl and glycidyl groups is cured in the presence of a catalyst for the purpose of controlling the refractive index of the cured product at will, curing of the xcex2-epithiopropyl group proceeds at a higher rate than that of the glycidyl group and the unreacted glycidyl group remaining in the cured product causes white turbidity and lowers the glass transition temperature as a result of phase separation. According to this invention, an acid anhydride curing agent is allowed to coexist in the system whereby the curing catalyst activates the acid anhydride group to initiate the curing reaction and lets the acid anhydride group to participate in the curing reaction together with the xcex2-epithiopropyl and glycidyl groups to give a cured product with good heat resistance or uniform transparency or both.
Furthermore, this invention makes it possible to obtain the target properties by increasing the latitude in controlling the viscosity of a composition by letting the three of glycidyl, xcex2-epithiopropyl and acid anhydride groups coexist and at the same time controlling the proportion of components A, B and C in the aforementioned range. That is, controlling the total of the glycidyl and xcex2-epithiopropyl groups at a level of 1.35 moles or more per 1 mole of the acid anhydride group has made it possible to eliminate the unreacted acid anhydride groups in the cured product thereby lowering the water absorption. In addition, controlling the xcex2-epithiopropyl group at a level of 0.5 mole per 1 mole of the acid anhydride group has made it possible to lower the proportion of the ester linkage in the cured product thereby lowering the water absorption (saturated water absorption at 85xc2x0 C. and 85% RH) to less than 1%.
On the other hand, coexistence of 0.5 mole or more of the glycidyl group per 1 mole of the acid anhydride group has caused cross linking to occur primarily at the thioester linkage thereby keeping the glass transition temperature from lowering. Furthermore, this invention is distinguished in that the coexistence of three functional groups makes it possible to maintain the glass transition temperature at the specified level without leaving the unreacted functional groups in the cured product and at the same time increase the refractive index and lower the water absorption even if a given composition contains up to 1.9 moles of the glycidyl group and up to 2.2 moles of the xcex2-epithiopropyl group per 1 mole of the acid anhydride group.
However, the water absorption increases when the total of the glycidyl and xcex2-epithiopropyl groups becomes less than 1.35 moles per 1 mole of the acid anhydride group while the unreacted glycidyl group remains in the cured product and the glass transition temperature falls when the total exceeds 3.5 moles. The glass transition temperature of the cured product falls when the glycidyl group is less than 0.5 mole or greater than 1.3 moles. In particular, when the glycidyl group is in excess of 1.9 moles, some remains unreacted in the cured product thereby lowering the glass transition temperature and lessening the effect of enhancing the refractive index. Lowering the water absorption of the cured product for which the xcex2-epithiopropyl group is intended is not accomplished sufficiently when the xcex2-epithiopropyl group is less than 0.5 mole while the cured product becomes brittle when the xcex2-epithiopropyl group exceeds 2.2 moles.
In the case where an epoxy compound containing only the glycidyl group is cured by an acid anhydride in the presence of a curing catalyst, the presence of 1.1-1.25 moles of the glycidyl group per 1 mole of the acid anhydride group results in the highest glass transition temperature and the lowest water absorption, although still in excess of 2%, but an amount outside this range causes the group to remain unreacted to a considerable extent thereby lowering the glass transition temperature. In the case where a compound containing only the xcex2-epithiopropyl group is cured by an acid anhydride, the thioester linkage increases in the cured product and the glass transition temperature is short of a satisfactory level. Moreover, the aforementioned cured products suffer from a narrow latitude in control of the refractive index as they are not related to a mixed system of compounds containing the glycidyl and xcex2-epithiopropyl groups.
As described above, by devising a resin composition comprising the four components A, B, C and D, this invention accomplishes a wide latitude in control of the viscosity of the composition and endows the cured product with both low water absorption and good heat resistance.
Moreover, the resin composition of this invention can be made to serve as a liquid resin molding material and what is required in this case is that the aforementioned four components A, B, C and D are essential and a mixture of the four is liquid without the use of a solvent. Since component A is usually solid at room temperature, it is necessary that component B and/or component C is liquid and a homogeneous composition of components A, B and C prepared by mixing under heat in the absence of a curing catalyst or component D is a viscous liquid at room temperature. As for the curing catalyst or component D, it is dissolved in the acid anhydride or separately dissolved in a small amount of a solvent and then mixed uniformly into the liquid composition. Or, the liquid composition may take the form of a dispersion of an adduct of a tertiary amine and an organic acid or a microencapsulated curing catalyst in a liquid mixture of components A, B and C.
The resin composition of this invention comprises the aforementioned four essential components A, B, C and D as the whole or principal portion of the resin constituents, but it is allowable to incorporate additives compatible with the resin constituents in a small amount, for example, 20 wt % or less, depending upon the mode of usage. Such additives include agents providing a variety of functions such as the aforementioned initiators, antioxidants, parting agents, silane coupling agents and flame retardants. In addition, fillers such as silica particles, rubber and pigments, thickeners, colorants and stress-lessening particles are dispersed in the resin composition of this invention to prepare a material for sealing semiconductors. In this case, the fillers can be added in a large amount, equal to or more than that of the resin composition of this invention. In the case where the resin composition of this invention containing fillers is used as a liquid resin molding material, it is satisfactory if the composition free of the fillers is liquid.
Still more, the resin composition of this invention can be made to serve as a resin composition for optical use. In this case, it is possible to improve the practical properties of the material by incorporation of additives such as known antioxidants and ultraviolet absorbers to the extent that the incorporation does not deteriorate the optical properties of the material. Moreover, it is possible to improve the release properties of the cured material from the mold by the use or addition of known external and/or internal mold release agents. In the case where the resin composition is used as an adhesive or coating, it is possible to add a solvent or a diluents for control of the viscosity or to add a silane coupling agent such as xcex3-glycidylpropyltrimethoxysilane or an agent providing close contact such as triazinethiol for improvement of the adhesion to the substrate. Addition of silicon- or fluorine-based surfactants is also allowed to control smoothness and uneven evaporation during coating.
A procedure desirable for curing the resin composition of this invention as a molding material is the following; an aromatic episulfide (component A) is mixed with a glycidyl ether compound having two or more glycidyl groups in its molecule (component B) in advance at room temperature or under heat to prepare mixture AB, an acid anhydride (component C) and a curing catalyst (component D) are separately mixed to prepare mixture CD, an additive such as an antioxidant, an ultraviolet absorber or a parting agent is dissolved or dispersed in either of mixture AB or CD as needed, and the two mixtures are blended immediately before use. A solution of the curing catalyst or component D prepared by separately dissolving it in a small amount of a solvent is added to the liquid composition or an adduct of a tertiary amine and an organic acid or a microencapsulated curing catalyst is dispersed in a liquid mixture of components A, B and C. Incorporation of the aforementioned additives and fillers can be performed simultaneously or separately when mixture AB is blended with mixture CD. The liquid resin molding material thus formulated is further kneaded and defoamed to give a homogeneous material.
As for the molding of the resin composition of this invention, a suitable method is sealing by coating, casting or potting.
The curing time is normally 1-60 hours and the curing temperature is 50-200xc2x0 C., preferably 80-180xc2x0 C. Upon completion of the curing, the material is annealed at a temperature lower than the curing temperature or at 50-180xc2x0 C. for a period of 10 minutes to 5 hours and this treatment is desirable for removing strain from the cured material.
In the preparation of an optical material from the resin composition of this invention, a desirable procedure is the following; an aromatic episulfide (component A) is mixed with a glycidyl compound (component B) in advance at room temperature or under heat, an acid anhydride (component C) and a curing catalyst (component D) are separately mixed, an additive such as an antioxidant, an ultraviolet absorber and a parting agent is dissolved in one of the mixtures better suited for dissolving the additive, and the two mixtures are blended immediately before use. The resulting composition is cast in a mold made of glass or metal, subjected to curing under heat and removed from the mold to give the target optical material. It is desirable to degas the raw materials and additives under reduced pressure before or after mixing For prevention of the generation of air bubbles during curing.
A variety of transparent optical materials are used in liquid crystal displays; for example, prisms and micro-lenses are formed by screen printing on light dispersing plates, light leading plates and prism sheets. The resin composition of this invention or an optical material prepared therefrom can be used in such applications.
The resin composition of this invention can also be applied as an adhesive or coating to optical lenses, prisms, optical fibers and optical films. The refractive index of common optical materials is mostly in the range of 1.5-1.62 (for example, 1.586 for polycarbonate, 1.51-1.57 for MS resin and 1.61 for polyarylate) and an adhesive or coating matched in refractive index to these materials is prepared and used to prevent reflection at the interface due to the difference in refractive index.
When the resin composition of this invention is used as an adhesive or coating for optical materials, components A, B and C are mixed in such a manner as to obtain a composition matching to the substrate in refractive index, a solvent or a diluent is added to adjust the viscosity, a silane coupling agent such as xcex3-glycidylpropyltrimethoxysilane or an agent for providing close contact such as triazinethiol is added to improve adhesion to the substrate, and a silicon- or fluorine-based surfactant is added to control smoothness and uneven evaporation during coating. The homogeneous resin composition thus prepared is applied to the substrate which is surface-treated as needed, the whole or a part of the solvent is removed by a suitable drying method and the remaining composition is heated at 50-200xc2x0 C. to effect curing either under pressure when used as an adhesive or as it is when used as a coating.
The resin composition of this invention can be used suitably as a sealing material for light emitting diodes (LED) on account of its transparency, low moisture absorption and good heat resistance. For sealing of LED, an LED device mounted on a metal or ceramic stem or a metal frame is covered by a method such as casting and transfer molding and sealed by heating.